The present invention relates to the continuous cladding of metal wire with a metal and, more particularly, to the atomization spraying of a uniform coating of metal onto a metal wire.
There are numerous known methods of cladding metal wire with a second metal. Throughout this application, the term metal wire will be used to mean both wire or thicker metal rod. Further, metal as used in reference to the atomization process of the present invention means pure elemental metals and various alloys thereof. The most widely known method of depositing a metal onto a metal wire is electroplating. Such process involves the steps of sizing and chemically cleaning the wire. A strike or thin metal interface of the coating metal is then deposited on the wire. For example, if the wire is steel and the coating metal is copper, a thin copper or nickel coating is deposited on the wire by an initial electroplating step to insure proper bonding of the copper coating to the wire. The final electroplating process is then accomplished wherein the copper coating of the desired thickness is deposited on the wire. There are several problems with such electroplating operations. Due to the electrochemical nature of the process, the wire to which the metal is applied must be extremely clean. The metal to be deposited is usually an elemental metal. Finally, the recent numerous governmental pollution control regulations on the electroplating industry require large investments in pollution control equipment to assure compliance.
Another method of depositing metal onto a wire involves the passing of the metal wire through a molten bath of the second coating metal. By controlling the speed of the wire, a desired amount of the second metal will adhere to the wire to form a clad metal wire. Problems with this method include the concentricity of the cladding metal on the wire and the adhesion of the cladding metal to the wire. Improper intial adhesion of the cladding metal to the wire is not a great problem where the wire is subsequently drawn down to a considerably lesser diameter whereby the adhesion of the cladding metal to the wire is improved by the combined pressure and temperature of the drawing process. But improper adhesion is a problem for clad wire where the final product is essentially the same diameter as the clad wire when exiting the process. (Only a final single die draw is necessary in such cases to smooth the outer surface when the initial adhesion is satisfactory).
In the manufacture of coated rod electrodes such as ground rods, the copper or stainless steel cladding of the steel rod is used for corrosion protection of the rod. Such rods are installed in the ground and are expected to have long service lives of many years. Accordingly, most applications require that such ground rods meet Underwriter's Laboratories (UL) specifications. These specifications require that the coating metal be applied to a thickness not less than a certain thickness (0.010 in. or 0.025 cm for certain metals) at any point on the rod. Accordingly it is desirable to have good control over the thickness of the coating metal. If the thickness of the coating metal varies, it must be applied in sufficient quantity to assure at least the minimum thickness at all locations on the rod. A final die drawing operation will not act to redistribute the coating metal to assure a uniform coating of a thickness to meet the specified requirements, but rather displaces a uniform amount of coating from all surfaces of the wire.
Another UL specification for such ground rods is that the coating metal must be sufficiently adhered to the rod such that no cracking of the coating occurs when the rod is bent through a certain angle (30.degree. in certain applications). To meet such specifications, good degree of bonding between the coating and the rod is required.
Other specifications for various products, but the basic needs for the process of the present invention is a process that would insure the controlled depositing of a metal, including alloys, onto a metal wire to form a concentric coating of the desired thickness onto the wire. Further, a high degree of metallurgical bonding between the coating and the wire is desired. The process should be adaptable to a continuous coating operation. The process should require a minimum of one final die drawing or cold working of the coated rod. Ideally only a single die operation to size the final diameter and insure the physical properties of the wire would be required. The process should be adaptable to a wire having a patterned surface such as a reinforcing bar wherein no final die drawing would be possible.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for the coating of a metal wire with a second metal.